A heatwave already punishing parts of the U.S. Southwest on Monday was expected to move into California this week, prompting the forecasters to warn of health and fire dangers.

A high-pressure ridge that built over southwestern deserts over the past few days is responsible for the unusually blistering heat this early in the year, National Weather Service meteorologist Karleisa Rogacheski said.

“Today last day of seasonable weather in California,” Rogacheski said.

California saw balmy weather on Monday, with temperatures in the upper 80s and low 90s Fahrenheit (30-35°C), but forecasts called for warming on Tuesday, spiking into the triple digits by Thursday and lasting several days.

The weather service issued an excessive heat warning for parts of southwest Arizona, including Phoenix, on Monday, predicting “dangerously hot conditions” at last through Saturday.

“Very High Heat Risk. Increase in heat-related illnesses, including heat cramps, heat exhaustion, and heat stroke. Heat stroke can lead to death,” the NWS said in the advisory.

California’s dry winter left forests and brush parched, prompting worries that the heat wave could touch off wildfires.

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